APOD: Green Flashes: Sun, Moon, Venus, Mercury (2020 May 30)

[APOD Robot]

Green Flashes: Sun, Moon, Venus, Mercury

Explanation: Follow a sunset on a clear day against a distant horizon and you might glimpse green just as the Sun disappears from view. The green flash is caused by refraction of light rays traveling to the eye over a long path through the atmosphere. Shorter wavelengths refract more strongly than longer redder wavelengths and the separation of colors lends a green hue to the last visible vestige of the solar disk. It's harder to see a green flash from the Moon, not to mention the diminutive disks of Venus and Mercury. But a telescope or telephoto lens and camera can help catch this tantalizing result of atmospheric refraction when the celestial bodies are near the horizon. From Sicily, the top panels were recorded on March 18, 2019 for the Sun and May 8, 2020 for the Moon. Also from the Mediterranean island, the bottom panels were shot during the twilight apparition of Venus and Mercury near the western horizon on May 24.

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[Ann]

Blue flash at Paranal. Photo: G. Blanchard/ESO

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Green, blue and purple flashes. Photo: Allen Vinson/Point Arena Lighthouse.

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You know me, I couldn't resist this, of course!

The lack of blue flashes in the APOD notwithstanding, the series of green flashes on all the major bodies of the inner Solar system still makes for a great APOD!

Ann

[Chris Peterson]

It's harder to see a green flash from the Moon[/url], not to mention the diminutive disks of Venus and Mercury. But a telescope or telephoto lens and camera can help catch this tantalizing result of atmospheric refraction when the celestial bodies are near the horizon.

Indeed. It is extraordinarily easy to see this with Venus, Mercury, or any star. In fact, it's a major annoyance when making color images or conducting photometry at low altitudes. Atmospheric dispersion spreads stars and planets into little rainbows. And it creates fringes around the Moon, red at the bottom and green to violet at the top. Which is really what we're seeing here, not green "flashes". Only the Sun produces a flash, because only the Sun is big enough and bright enough to disappear below the horizon almost entirely, leaving just its most refracted component visible.

[orin stepanek]

I'm speechless! I can't think of anything to say about flashes!

[Joe Adlhoch]

Very cool pictures. But with the Venus and Mercury images, how do we know the green portion is due to the atmosphere and not the telescope/eyepiece?

[Chris Peterson]

Very cool pictures. But with the Venus and Mercury images, how do we know the green portion is due to the atmosphere and not the telescope/eyepiece?

There's no reason that the dispersion from optics would be linear like this, or perpendicular to the horizon like this.

[ngc1535]

Chris,

Atmospheric dispersion causes the chromatic fringing as you describe above.
However, a flash is the refracted/broken/distorted/separated image of the fringe- which you got close to saying above.
A "flash" therefore does not need to be above the horizon while the fringe/rim of the source is below the horizon- nor is the effect limited to the Sun.

So in the APOD picture the Sun and the moon show "flashes" the other two pictures do not- just the fringes from normal dispersion.

-adam

[Chris Peterson]

Chris,

Atmospheric dispersion causes the chromatic fringing as you describe above.
However, a flash is the refracted/broken/distorted/separated image of the fringe- which you got close to saying above.
A "flash" therefore does not need to be above the horizon while the fringe/rim of the source is below the horizon- nor is the effect limited to the Sun.

So in the APOD picture the Sun and the moon show "flashes" the other two pictures do not- just the fringes from normal dispersion.

-adam

I guess it's a matter of definition. I'm not sure I'd call a simple detachment a "flash", but it's not really that important. My point was primarily with the two planet images and the assessment of capturing that being "hard". I've got dozens of images that show this effect on stars. I wish it was hard! What it actually is is "hard to avoid".

(To be clear, I wasn't criticizing the APOD itself, which shows an interesting effect in an interesting way. Just taking exception with a detail of the caption.)